There is a need for a simple, low power, light-weight, compact, high specific impulse electric propulsion device to satisfy mission requirements for micro and nano-satellite class missions. Satisfying these requirements entails addressing the general problem of generating a sufficiently dense plasma within a relatively small volume and then accelerating it in a way that generates a net thrust reaction force in a desired linear direction. Known means for ion generation and propulsion generally require relatively large containment volumes in order to achieve reasonable ionization efficiencies, therefore new means are needed in order to achieve effective scaled-down propulsion devices.
Recent prior art electric propulsion devices and plasma accelerators are commonly some form of Hall effect thrusters (Hall accelerators or Hall engines). A conventional Hall effect thruster generally comprises an accelerating channel arranged along an axis with an anode and a propellant source at a first, generally closed, end of the channel, and a cathode (electron source) at a second, generally open, end of the channel. The cathode and anode establish an electric field with a gradient generally aligned with the axis of the channel. A system of magnets is arranged so that a magnetic field crosses the channel.
To continue the description of the Hall effect thruster, an exemplary thruster is presented comprising an annular accelerating channel extending circumferentially around the axis of the thruster and also extending in an axial direction from a closed end to an open end. The anode is usually located at the closed end of the channel, and the cathode is positioned outside the channel close to its open end. Means is provided for introducing a propellant, for example xenon gas, into the channel and this is often done through passages formed in the anode itself or close to the anode. A magnetic system applies a magnetic field in the radial direction across the channel and this causes electrons emitted from the cathode to move circumferentially around the channel. Some but not all of the electrons emitted from the cathode pass into the channel and are attracted down the electric field gradient towards the anode. The radial magnetic field deflects the electrons in a circumferential direction so that they move in a spiral trajectory, accumulating energy as they gradually drift towards the anode. In a region close to the anode the electrons, collide with atoms of the propellant, causing ionization. The resulting positively charged ions are accelerated by the electric field towards the open end of the channel, from which they are expelled at great velocity, thereby producing the desired thrust. Because the ions have a much greater mass than the electrons, they are not so readily influenced by the magnetic field and their direction of acceleration is therefore primarily axial rather than circumferential with respect to the channel. The ion stream is at least partially neutralized by those electrons from the cathode that do not pass into the channel.
Conventionally, the required radial magnetic field has been applied across the channel using an electromagnet having a yoke of magnetic material which defines poles on opposite sides of the channel, i.e. one radially inwardly with respect to the channel and the other radially outwardly with respect to the channel. An example is shown in European patent specification 0 463 408 which shows a magnetic yoke having a single cylindrical portion passing through the middle of the annular channel and carrying a single magnetizing coil; and a number of outer cylindrical members spaced around the outside of the accelerating channel and carrying their own outer coils. The inner and outer cylindrical members are bolted to a magnetic back plate so as to form a single magnetic yoke.
A recent example of the Hall effect thruster is disclosed in U.S. Pat. No. 5,847,493 (Yashnov, et al.; 1998) entitled “Hall Effect Plasma Accelerator”. The described invention in the U.S. Pat. No. 5,847,493 comprises the use of magnets (permanent or preferably electric) wherein the magnetic poles are defined on bodies of material which are magnetically separate in order to allow greater freedom in selecting the dimensions of the thruster, particularly the length in the axial direction relative to the diameter of the accelerating channel.
U.S. Pat. No. 5,751,113 (Yashnov, et al.; 1998), discloses a closed electron drift Hall effect plasma accelerator with all magnetic sources located to the rear of the anode. It is stated that this makes it possible to provide a Hall effect accelerator with an optimum distribution of magnetic field inside the acceleration channel by means of a simpler and less heavy arrangement using a single source of magnetic field, such as a single coil or permanent magnet. As in all Hall effect thrusters, the magnetic field lines (13, as seen in FIG. 2) extend laterally across the accelerating channel (1) over the anode (2) and propellant gas source (3) located at the closed end of the channel (see FIG. 1).
A problem common to the Hall effect thrusters is one of scaling its size. In general, it is difficult to scale down Hall effect thrusters appreciably because of the magnetic field requirements. In smaller engines, the large transverse magnetic fields required can hamper ion flow, thereby reducing the ion beam current. This is particularly problematic for such engines generating milliamp magnitude beams for micro-thruster applications, wherein small thrust to power ratios make Hall effect thrusters impractical for micro-satellite applications. Another scaling problem is that electromagnets do not scale well with size reduction because of heating issues and coil size required to achieve the desired field.
Hall effect thrusters generally employ hollow cathodes, and preferably employ electromagnets, thereby requiring fairly complicated, and thus heavier, control systems in order to control electromagnet current, gas flow in both the anode and the discharge electrode, and cathode discharge current. Adding to the problems of complexity and weight, the hollow cathode consumes propellant.
U.S. Pat. No. 6,075,321 (Hruby; 2000), discloses a Hall field plasma accelerator with an inner and outer anode, designed to deal with problems of wall heating and sputtering that are characteristic problems with Hall effect thrusters.
A non-Hall effect thruster is described by U.S. Pat. No. 4,937,456 (Grim, et al.; 1990), that discloses a dielectric coated ion thruster comprising a cathode chamber (12) from which free electrons flow into an attached ionization chamber (14) along with a flow of ionizable gas atoms. According to the abstract and to column 6 of the detailed description, the free electrons are accelerated by a positive potential applied to the interior surface of the ionization chamber, causing the electrons to collide with atoms of the gas with sufficient kinetic energy to create ions. The positively charged ions are accelerated toward a negatively charged perforated grid plate (24, 112), pass through the grid plate, and exit in a focused beam, providing thrust in the opposite direction. A plurality of bar magnets (20, 22, 108, 110) are arranged in a spaced apart circular array around the cathode chamber with a pole face of each of the magnets tangentially aligned with wall sections (16, 18, 102, 104) of the ionization chamber. The bar magnets define an axial geodesic picket fence arrangement that extends circularly about the cathode chamber, wherein the pole faces of adjacent bar magnets that are in contact with the ionization chamber alternate north and south polarity, so that a magnetic field extends between the opposite pole faces of adjacent bar magnets. Although magnetic field lines are not illustrated, it can be seen from FIGS. 1 and 5, for example, that the magnetic field lines will arch from pole to pole to create a scalloped line around the circumference of the ionization chamber with cusps occurring at each pole. As stated in column 7 of the detailed description, as a negatively charged electron is accelerated toward the wall sections, the magnetic field interacts with the moving charge, causing the electron to experience a force directed generally at a right angle to its forward velocity. In response to this force, the electrons are caused to spiral in a helical path, thereby extending the mean path of the electrons to increase the probability that the electrons may strike an atom and ionize it. Since the magnetic field lines that confine the plasma within the ionization chamber bend laterally away from the magnet poles (forming cusps), the surfaces of the poles are not well protected by the magnetic field and would normally be exposed to erosion due to impacts by high-energy electrons or ions, therefore dielectric coating (42) is provided to protect them from sputtering. Likewise, the outer surface of an emitter tube (28, 61, 128), and the inner and outer surfaces of the grid plate, are coated with a dielectric material to protect them from sputtering erosion.
Problems inherent in conventional ion thrusters with grids (e.g., U.S. Pat. No. 4,937,456) include significant erosion issues for which dielectric coatings are needed to help provide protection, thereby adding weight and complexity. Furthermore, the use of grids along with charged chamber walls require the use of multiple power supplies, thereby complicating the power processor unit. Finally, gridded systems have inherently lower thrust density capability relative to gridless concepts.
It is known that plasma accelerators can be used for material processing in a vacuum by means of plasma ion interaction with materials. U.S. Pat. No. 6,380,684 (Li, et al.; 2002) discloses a plasma generating apparatus and semiconductor manufacturing method which generates a high-density plasma in a rectangular chamber using magnetron, high frequency discharge plasma generation, i.e., a high frequency oscillating electric field that interacts with magnetic fields to produce electrons and ions in a plasma. An annular-rectangular (“fistulous”) discharge electrode (14) is in close proximity to concentric annular-rectangular permanent magnets (15,16) that are arranged axially on either side of the discharge electrode to generate magnetic field lines that loop over the discharge electrode to cusps that are on either axial side of the electrode. Rectangular parallel plate electrodes (17, 18) at the top and bottom of the chamber are either grounded or connected to a second high frequency source. The top electrode 17 is used, for example, as gas diffusion plate for diffusing a discharge gas or a process gas, wherein the top electrode (17) is a perforated gas shower plate (37).
It is an object of the present invention to provide a compact plasma accelerator that overcomes problems such as those described hereinabove for known devices, thereby providing sufficient thrust density to provide a simple, low power, light-weight, compact, high specific impulse electric propulsion device to satisfy mission requirements for micro and nano-satellite class missions.